1. Field of the Invention
The present invention relates to a pressure control system for use with an automatic transmission, and, more particularly, to such a system in which a coupling pressure, supplied to frictional elements of the automatic transmission during gear shifting, is regulated based on a control pressure output from a control-pressure regulating valve during the gear shifting, and is regulated by a line pressure after completion of the gear shifting.
2. Description of the Related Art
Generally, a clutch and a brake, corresponding to frictional elements of an automatic transmission, are devices to realize restrictions for attaining desired gear positions during the traveling of a vehicle. The clutch and brake are usually operated or released by the use of a hydraulic pressure.
To prevent slippage during gear shifting, the frictional elements must receive a maximum hydraulic pressure corresponding to a torque of the respective frictional elements based on a maximum input torque per each gear position. Also, the frictional elements must receive an accurately controlled hydraulic pressure belonging to an appropriate hydraulic pressure control range (hereinafter, referred to as a coupling pressure), for the sake of a smooth shifting feeling. In a conventional system, the control pressure is generally regulated by a control pressure regulating valve.
However, conventional pressure control systems for use with an automatic transmission frequently have a problem in that the control pressure of the control-pressure regulating valve has a limited control range since the hydraulic pressure control range outputted during the gear shifting is less than the maximum pressure. Further, since the coupling pressure has a high sensibility for the variation of the control pressure of the control-pressure regulating valve, shift feeling largely varies in accordance with a deviation of the control pressure of the control-pressure regulating valve.
Moreover, when the torque of the respective frictional elements per each gear position largely varies as like a six-speed automatic transmission, there can be a technical problem in that the hydraulic pressure control range outputted during the gear shifting is extremely limited to approximately 20 to 30% of the maximum pressure.
As a result, in the case of gear shifting from a gear position having a low torque, the displacement of the coupling pressure in accordance with the variation of the control pressure of the control-pressure regulating valve. inevitably increases due to the narrow control pressure range of the control-pressure regulating valve. That is, a coupling-pressure regulating valve can exhibit a high sensibility for an input to output ratio, causing a shock during the gear shifting.
For example, as shown in FIG. 1, when the control pressure range of the proportional-control solenoid valve is approximately 1.5 bar to 5.5 bar, and the coupling pressure to be supplied from the coupling-pressure regulating valve to a corresponding frictional element is regulated to a range of approximately 0 bar to 5 bar, an actual control range X of the control-pressure regulating valve is limited to approximately 1.5 bar to 3.5 bar.
Accordingly, the coupling pressure to be supplied from the coupling-pressure regulating valve to the corresponding frictional element largely varies based on the control pressure supplied from the proportional-control solenoid valve to the coupling-pressure regulating valve, resulting in an increase in the sensibility of gear shifting.